The present invention relates to an image forming apparatus such as a digital copying machine, which has an automatic double-side unit and can effect printing on both sides of a paper sheet, and to an image forming method.
In a conventional digital copying machine, when a copying operation is executed, an original is fed to a reading mechanism and paper is fed to a printing/outputting mechanism. Thus, the copying is performed. In the case of double-side printing, a paper sheet, on one side of which printing has been effected, is reversed by a reversing section of an automatic double-side unit, and the paper sheet is fed once again.
In particular, in an automatic double-side unit with stackless configuration, the time difference between the first side recording of a paper sheet and the second side recording of the sheet has become shorter (e.g. within 10 seconds because of higher operation speed). In addition, with development in reduction of apparatus size, the distance of conveyance of the sheet between the completion of the first side recording and the start of the second side recording has become shorter. Consequently, since the time during which the sheet is set aside from the high-temperature section is short, it is difficult to sufficiently cool the sheet.
It is confirmed that when double-side printing (recording) is performed in the above-described circumstances, there occurs a difference in magnification of 0.2% to 0.4% between an image formed on an obverse side (first surface) of a sheet and an image formed on a reverse side (second surface) of the sheet (data on ordinary paper with a thickness of about 0.080 mm).
The situation below may be considered (numerical values are assumed ones).
Assume that the temperature of the paper sheet at the time of first surface recording is 30° C. and the width of the sheet is W (mm). When the first surface of the sheet is subjected to fixation by a fixing device, heat is sufficiently accumulated in the sheet at the fixing temperature (e.g. 160° C.). As a result, the sheet contracts.
Then, the sheet is reversed by the automatic double-side unit, and printing is effected on the second surface. At this time, the temperature of the sheet is 100° C., and the width of the sheet has not fully recovered from thermal contraction. The width is about 0.997W (mm).
Assume that a line segment of 280 mm is recorded on each of both surfaces of the sheet in the main-scan direction.
In the state in which the sheet is cooled and its sized has recovered to 100% from thermal contraction, the length of the recorded line segment on the first surface of the sheet is 280 mm and the length of the recorded line segment on the second surface is 280/0.997=280.84 mm.
In the prior art, it has been thought that there is no need to individually set magnifications in the main-scan direction and sub-scan direction with respect to the obverse surface and reverse surface of a paper sheet in the case of double-side printing (recording).
However, if numerical values as in the above-described example are indicated, there is such a problem that the main-scan magnification and sub-scan magnification need to be adjusted to proper values in the case of double-side printing in accordance with the structure and specifications of the apparatus, thereby to ensure a print position and dimensional precision.